The long-term goal of this project is to understand the molecular network through which brassinosteroid (BR) and other hormonal and environmental signals together control plant growth and development. Steroids are important hormones in both plants and animals. BR regulates a wide range of developmental and physiological processes in plants through a signal transduction pathway from cell- surface receptor kinases (BRI1) to nuclear transcription factors (BZR1 and BZR2/BES1). BR acts together with other hormonal and environmental signals to regulate plant development and physiology, and has particularly intimate relationships with light and gibberellin (GA). We have made great progress in understanding the molecular details of how BR binding to BRI1 leads to activation of BZR1 and regulation of over a thousand target genes. Furthermore, we have gained insights into how the BR signaling pathway is wired together with other pathways to control various functions in plants. For this renewal proposal, we plan to understand how the BR signaling pathway crosstalks with the stomata receptor kinase pathway and the light and gibberellin signaling pathways to coordinate cell differentiation and plant development. We will use a combination of genetic, proteomic, and genomic approaches to achieve the following specific Aims. 1) We will study how BR signaling regulates the MAP kinases and crosstalks with the ERECTA family receptor kinases to regulate stomata differentiation. 2) We will elucidate the mechanism of integration of the BR and light signaling pathways, by characterizing the interaction between BZR1 and the phytochrome- interacting factors (PIFs) in regulating common target genes, and the functions of three BZR1-target transcription factors in light regulation of gene expression and development. 3) We will study the molecular mechanisms by which BR and GA hormones orchestrate common developmental processes. The experiments proposed in this proposal will elucidate a regulatory network that integrates four important signaling pathways controlling plant growth. This will greatly advance our understanding of not only the molecular mechanisms of signal transduction but also the mechanisms of signaling pathway integration. Although the BR signaling pathway is distinct from the nuclear receptor-mediated steroid signaling mechanism in animals, many components and signaling mechanisms used by the BR pathway are conserved and play important roles in either steroid regulated processes or other signaling pathways in animals. Thus, this study not only is important for plant biology and agriculture, but also can potentially help us understand fundamental mechanisms of steroid function and cellular regulation that are relevant to human health.